Trocars are pointed surgical instruments which puncture tissue to obtain access to body cavities. Inherent with such instruments is the danger that after the intended puncture, the trocar will inadvertently puncture unintended tissue, vessels or organs.
The prior art teaches various techniques for providing needles and trocars with means to avoid inadvertent puncturing. For example, hollow Veress needles are provided with spring-loaded internal pins which pop out beyond the sharp cutting end of the needles after the intended puncture has been completed. A more recent type of shielding used with trocars may be seen in U.S. Pat. Nos. 4,601,710 and 4,654,030, wherein the shielding takes the form of a sleeve which extends around the point of the trocar after piercing is complete. Both of these techniques require that something project beyond the piercing point of the needle or trocar, and require the addition of a separate shielding element.
The prior art also teaches trocars which are provided with trocar tubes received around the trocar so as to leave a passage extending into the body cavity after the trocar is removed. U.S. Pat. Nos. 4,601,710 and 4,654,030 both teach such tubes. In the case of these patents, however, a shielding sleeve, in addition to the tubes, is required. This necessarily increases the bulk of the trocar and adds to the complexity of its mechanism. Projecting a shielding sleeve beyond the end of the trocar also, necessarily, results in an additional element being extended beyond the tip of the trocar and into the body cavity. While this sleeve performs a shielding function while within the cavity, it would be ideal if the pointed tip of the trocar could be shielded without the necessity of extending yet another element into the body cavity.
U.S. Pat. No. 4,535,773 teaches techniques for shielding the sharp tip of a trocar through either the interposition of an extendable shielding sleeve, or the retraction of the trocar into the trocar tube. The latter arrangement is seen in the embodiment of FIGS. 34 and 35 of the patent. It relies upon a solenoid-operated detent which holds the trocar in the extended position relative to the trocar tube and requires electronic sensing means in the tip of the trocar to activate the detent for release. This sensing means requires an electronic alarm network.